5.2 - Characteristics of Rivers & Valleys
Changes in river characteristics from source to sea
Rivers undergo significant transformations as they flow from their upland origins to the sea. These changes influence the shape of the river channel, the volume of water, and the energy available for shaping the landscape.
Key transformations in river features
- Channel width - The river channel starts narrow near the source and gradually widens as it approaches the sea due to the joining of tributaries.
- Discharge levels - Discharge, which is the volume of water flowing through the river per second, increases downstream as smaller streams and rivers contribute to the flow.
- Velocity and energy - A river's energy is determined by its velocity, or speed of flow. High velocity, often seen during floods or on steep gradients, results in greater energy to shape the landscape.
- Load transportation - With high energy, rivers erode the channel bed and banks, carrying materials like sand and stones as load. When energy decreases, this load is deposited on the bed or banks.
The cross profile of river valleys at different stages
The cross profile of a river valley, which shows its shape when viewed from the side, evolves as the river progresses from its source to the sea. This shape is influenced by the river's energy and the processes acting on the surrounding land.
Valley shapes across river stages
- Upper stage - Near the source, the valley has a narrow floor with steep, almost vertical sides, forming a distinct V-shape due to intense downward erosion.
- Middle stage - Further downstream, the valley floor becomes wider, and the sides slope more gently as lateral erosion starts to dominate.
- Lower stage - Close to the sea, the valley features a very wide floor with gentle sides, often accompanied by a broad floodplain where the river can spread out during high flow.
The long profile of a river and its gradient variations
The long profile of a river represents its path from source to sea when viewed from the side, showing how the gradient, or steepness, changes over the course of its journey. This profile affects the river's energy and erosion capabilities.
Gradient changes along the river's course
- Upper stage gradient - At the start, near the source, the river has a steep gradient, providing high energy for erosion and transport of materials.
- Middle stage gradient - As the river progresses, the gradient becomes more gentle, reducing the energy available but allowing for wider channel development.
- Lower stage gradient - Near the sea, the gradient is very gentle, almost flat, leading to deposition of materials and the formation of features like floodplains.
The processes of river erosion and their impact on the landscape
Erosion is the process by which rivers wear away the land, shaping valleys and altering the river channel. There are four primary methods of erosion, each contributing to the landscape in unique ways.
Types of river erosion processes
- Abrasion - This occurs when the river's load, including stones and sand, scrapes against the bed and banks, wearing them down. During high-energy events like floods, this can create features such as potholes, where swirling materials enlarge depressions in the riverbed.
- Attrition - The load particles themselves are eroded through collisions with the riverbed or each other, breaking into smaller, smoother pieces over time.
- Hydraulic action - The sheer force of the water erodes softer materials like clay and weakens rocks by exploiting cracks and joints, gradually breaking them apart.
- Corrosion - Certain rocks, such as chalk and limestone, dissolve in the river water, contributing to the gradual wearing away of the channel over time.